The production of organic chemicals by microorganisms is well known to those familiar with the fermentation art. Such fermentation reactions frequently produce a variety of products in dilute aqueous solutions. The expense of separating the chemicals from each other and from the large volumes of water has been so great that production of chemicals by fermentation has not been able to compete with the production of the same chemicals from fossil fuel sources. However, the gradual depletion of petroleum fossil fuel with a resultant increase in prices of petrochemical feedstocks has revived interest in fermentation reactions which can convert renewable raw materials into simple organic chemicals.
Fermentations that produce a single product are particularly desirable since product isolation from such reactions is simplified. Certain microorganisms, known as homoacidogens, can be used in such a procedure to give a single acid when grown on a variety of hexoses, pentoses and lactic acid. The fermentation of a hexose such as glucose by Clostridium thermoaceticum, hereafter abbreviated C. thermoaceticum. is especially attractive since it can produce theoretically 3 moles of acetic acid from 1 mole of the sugar.
C. thermoaceticum was first isolated by Fontaine, et al, J. Bacteriol., 43, 701-715 (1942). The wild strain is an anaerobe that grows best at a pH of about 7. Its growth is inhibited by low pH, acetic acid, and acetate. For these reasons, this strain does not produce acetic acid in high concentration.
Various workers have attempted to obtain mutant strains of C. thermoaceticum that would produce higher concentrations of acetic acid in a fermentation reaction. Wang, et al, AIChE Symp. Ser. 181, 74, 105-110 (1978), described an improved strain of the microorganism that had a higher tolerance for sodium acetate. Schwartz and Keller, Applied and Environmental Microbiology, 43, 117-123 (1982), reported isolation of a strain of C. thermoaceticum capable of growing and producing acetic acid at a pH of 4.5. This work is also disclosed in European patent application No. 81104811.5, published June 1, 1982.
We have now isolated a new mutant of C. thermoaceticum, capable of growing at a pH of less than 5, which is a more efficient producer of acetic acid than any of the strains previously reported.